Based on the thermal decomposition of nitrogen pentoxide (N2O5) and the thermal balance relationship between N2O5 and NO3 free radical, the concentrations of N2O5 and NO3 free radical are measured by cavity ring-down spectral technology. In view of the reversible balance between NO2 and N2O5, the influences of heating temperature and NO2 concentration on decomposition rate of N2O5 are investigated. Considering the loss of N2O5 in the measurement system, the inlet efficiency obtained by the preliminary quantitative analysis is about 88%. The optimum integration time is chosen according to the Allan variance, and the detection limit of volume fraction for the out-field observation is 8.6×10 -12. Considering the uncertainty of the inlet efficiency, absorption cross section and incompletion thermal decomposition of N2O5, the estimated total measuring error is about ±10%. A practical detection experiment for nocturnal atmosphere is carried out in Hefei. The N2O5 concentration ranges from 3.5×10 -11 to 1×10 -9 with a average value of 4.52×10 -10 during the measurement period. The proposed technique provides efficient way for realizing high sensivity on-line monitoring of N2O5 and NO3 free radical in ambient air.